An example of such a spectrometer arrangement is an echelle spectrometer with internal order separation. The problem to which the invention is directed will be explained in the following based on an echelle spectrometer.
Such a spectrometer arrangement is known, for example, from DE 10 2009 059 280 A1.
In the case of an echelle spectrometer, the gratings have a stepped cross section. Illuminating the short facet of the step-like structure with a corresponding blaze angle produces a diffraction pattern, which concentrates the diffracted intensity in high orders, e.g. in the fiftieth to one hundredth orders. In this way, high spectral resolutions can be achieved in compact arrangements. The orders can—, depending on occurring wavelengths—superimpose. The orders are in the case of echelle spectrometers dispersed with internal order separation, consequently a second time, crosswise to the dispersion direction of the echelle grating, in order to separate the different orders. One obtains, in this way, a two-dimensional spectrum, which can be registered with detector arrays.
An echelle spectrometer with internal order separation differs from echelle spectrometers with external order separation in that in the latter case only radiation from a small spectral region enters into the spectrometer. In the case of spectrometers with internal order separation, the spectrum is produced in the form of a two-dimensional structure in the detector plane. This structure is composed of spectral sections arranged essentially in parallel with one another. The free spectral regions of the particular diffraction orders give, together, a gapless spectrum for a certain wavelength range. The application of a detector array with a large number of detector elements permits the simultaneous registering of a large wavelength range with high spectral resolution.
In the case of spectrometer arrangements, the quality of the spectrum produced in the image area is limited by various imaging errors, especially by astigmatism, coma and/or spherical aberration. If the radiation of a wavelength in the case of the imaging of a point source is not concentrated in a single image point on the detector, but distributed on a larger area in the image plane, this degrades the spectral resolution of the spectrometer by broadening the image point in the dispersion direction and/or leads to a worsened signal-noise ratio by broadening the image point crosswise to the dispersion direction. In the special case of an echelle spectrograph, wherein a plurality of diffraction orders are imaged next to one another, imaging errors can be brought about, which lead to a broadening of the image point transversely to the main dispersion direction and an overlap between neighboring diffraction orders.